You can set restrict the country field within an address question so that only 1 country is selected. To enable this setting, you can enable "Restrict addresses to a country". Here's how:
Navigate to the form editor.
Locate the address field you'd like to add a default country to.
Hover over the field, and click on the cogwheel icon appearing on the right.
A popup will appear containing configuration options for your field. Scroll until you find one labeled "Restrict addresses to a country".
A dropdown selection will appear. Select the country you wish to use from the list provided.